John Galt is Prometheus who changed his mind. After centuries of being torn by vultures in payment for having brought men the fire of the gods, he broke his chains and he withdrew his fire—until the day men withdraw their vultures. (Part II, Chapter V)
Prometheus was the Titan god of forethought and crafty counsel who was entrusted with the task of molding mankind out of clay. His attempts to better the lives of his creation brought him into direct conflict with Zeus. Firstly he tricked the gods out of the best portion of the sacrificial feast, acquiring the meat for the feasting of man. Then, when Zeus withheld fire, he stole it from heaven and delivered it to mortal kind hidden inside a fennel-stalk. As punishment for these rebellious acts, Zeus ordered the creation of Pandora (the first woman) as a means to deliver misfortune into the house of man, or as a way to cheat mankind of the company of the good spirits. Prometheus meanwhile, was arrested and bound to a stake on Mount Kaukasos where an eagle was set to feed upon his ever-regenerating liver (or, some say, heart). Generations later the great hero Herakles came along and released the old Titan from his torture.
In Francisco’s comment, Prometheus (personified by Galt) represents the great industrialists who have provided men with prosperity and improved their lives with their inventions and products, but have received only condemnation and government interference in return. These men, led by Galt, have disappeared and taken their prosperity-generating minds (the “fire” they had provided) with them. They will no longer allow themselves to receive torture as payment for their talents, and they will only return their talents to the world when they are no longer punished for bringing them.